The present invention relates to ultrasonic generators and is concerned in particular with such generators used in ultrasonic cleaning systems.
Ultrasonic cleaning systems employ one or more transducers mounted on or within a liquid-containing chamber into which articles to be cleaned are placed. The liquid is usually water dosed with suitable chemicals, such as soaps and detergents, for washing the articles in the chamber. To assist in dislodging dirt, oil, grease etc. from the articles the transducer is caused to transmit ultrasonic vibrations to the articles via the liquid.
Conventional ultrasonic transducers fall into two principal types. The first type, which has been used for the last thirty years or so, is the magnetostrictive variety which employs a stack of metal laminations carrying a coil which is electrically excited by an alternating current. The laminations are arranged to be mechanically coupled to the body of a liquid-containing tank whereby ultrasonic vibrations generated in the laminated stack are transferred to articles placed in the tank via the tank body and the liquid.
A more recent type of ultrasonic transducer operates on the piezo-electric principle and comprises one or more piezo discs bonded to the tank via a metal diaphragm plate and adapted to be subjected to a high level alternating voltage.
The operating characteristics of these two types of device are somewhat different. Piezo electric elements are essentially capacitive devices and can be driven by very high voltage waves. Because of their high deflection capability and because they are voltage operated, piezo-transducers can be driven efficiently off mechanical resonance by using higher drive voltages than would be needed at resonance. Although this imposes greater electrical stress on the piezo devices, it does not significantly reduce their operational efficiency.
In contrast, magnetostrictive transducers are essentially current operated devices and their operation is limited by I2R losses in the windings. Furthermore, they have a very much smaller vibration amplitude limit than piezo transducers and greater losses within the driving material. Thus, efficiencies of the order of 50% have to be tolerated in comparison with about 90% for a piezo system.
The piezo transducer is, however, at a mechanical disadvantage. Failure of the piezo-elements themselves, failure of the epoxy bond to the diaphragm plate and erosion of the diaphragm plate itself, are all common, whereas magnetostrictive transducers are exceedingly strong and reliable devices capable of a long operational life in arduous conditions without failure. Thus, there is a high incentive to utilise magnetrostrictive transducers where possible.
However, magnetostrictive transducers have another associated problem in practice which has traditionally reduced their effectiveness. In order to achieve the necessary cleaning power, a number of magnetostrictive transducers are usually mounted together on a common diaphragm plate. For example, one diaphragm plate might carry two banks of transducers, with five series connected transducers in each bank. A serious problem which results arises from the variation in resonant frequencies between one stack and the next and between the individual transducers in each bank caused by the inevitable slight differences in the mechanical characteristics of each transducer and in, for example, the thickness of the brazing used to attach them to the diaphragm plate and in the relative positions of the transducers on the diaphragm plate which results in different loadings.
The traditional means of operating such transducers has been to drive them with a low-level oscillator pre-set to a fixed frequency that gives the peak power, as measured by input current to the system. However, in operation of such a system, the mechanical dimensions of the transducer assembly will inevitably vary, for example due to temperature variations, and the resonant condition is consequently lost, with a corresponding drop in operating efficiency. In consequence, the overall performance is particularly poor in comparison with a conventional piezo system.